Cold rolled and annealed steel sheet and method of manufacturing the same

ABSTRACT

A cold-rolled and heat-treated steel sheet having a microstructure of, in surface fraction:
         between 10% and 30% of retained austenite, said retained austenite being present as films having an aspect ratio of at least 3 and as Martensite Austenite islands, less than 8% of such Martensite Austenite islands having a size above 0.5 μm,   at most 10% of fresh martensite and   recovered martensite containing precipitates of at least one element chosen among niobium, titanium and vanadium. A manufacturing method thereof is also provided.

The present invention relates to a high strength steel sheet having highductility and formability and to a method to obtain such steel sheet.

BACKGROUND

To manufacture various items such as parts of body structural membersand body panels for automotive vehicles, it is known to use sheets madeof DP (Dual Phase) steels or TRIP (Transformation Induced Plasticity)steels.

SUMMARY OF THE INVENTION

To reduce the weight of the automotive in order to improve their fuelefficiency in view of the global environmental conservation, it isdesirable to have sheets having improved yield and tensile strengths.But such sheets must also have a good ductility and a good formabilityand more specifically a good stretch flangeability.

It is an object of the present invention to provide a steel sheetreaching a yield strength of at least 700 MPa, a tensile strength of atleast 900 MPa, a uniform elongation of at least 12% and a hole expansionratio of at least 20%.

The present invention provides a cold-rolled and heat-treated steelsheet, made of a steel having a composition comprising, by weightpercent:

-   -   C: 0.03-0.25%    -   Mn: 3.5-8%    -   Si: 0.1-2.0%    -   Al: 0.03-2.0%    -   Ti≤0.080%    -   Nb≤0.080%    -   V≤0.2%    -   V+Ti+Nb>0.01%    -   S≤0.010%    -   P≤0.020%    -   N≤0.008%        and comprising optionally one or more of the following elements,        in weight percentage:    -   Mo: 0.1-0.5%    -   Cr: 0.01-1%    -   B: 0.0005-0.004%        the remainder of the composition being iron and unavoidable        impurities resulting from the smelting,        said cold-rolled steel sheet having a microstructure consisting        of, in surface fraction:    -   between 10% and 30% of retained austenite, said retained        austenite being present as films having an aspect ratio of at        least 3 and as Martensite Austenite islands, less than 8% of        such Martensite Austenite islands having a size above 0.5 μm,    -   at most 10% of fresh martensite and    -   recovered martensite containing precipitates of at least one        element chosen among niobium, titanium and vanadium.

DETAILED DESCRIPTION

The invention will now be described in details and illustrated byexamples without introducing limitations.

Hereinafter, Ae1 designates the equilibrium transformation temperaturebelow which austenite is completely unstable, Ae3 designates theequilibrium transformation temperature above which austenite iscompletely stable, Ar3 designates the temperature until which themicrostructure remains fully austenitic upon cooling, TΘ designates thetemperature above which cementite gets dissolved upon heating and Msdesignates the martensite start temperature, i.e. the temperature atwhich the austenite begins to transform into martensite upon cooling.

All compositional percentages are given in weight percent (wt. %),unless indicated otherwise.

The composition of the steel according to the invention comprises, byweight percent:

-   -   0.03%≤C≤0.25% for ensuring a satisfactory strength and improving        the stability of the retained austenite which is necessary to        obtain a sufficient elongation. Preferably, the carbon content        is higher than or equal to 0.1%. If the carbon content is too        high, the hot rolled sheet is too hard to cold roll and the        weldability is insufficient. If the carbon content is below        0.03%, the tensile strength will not reach the targeted values.    -   3.5%≤Mn≤8% for ensuring a satisfactory strength and achieving        stabilization of at least part of the austenite, to obtain a        sufficient elongation. Below 3.5%, the final structure comprises        an insufficient retained austenite fraction, and an insufficient        Mn content in the retained austenite, so that the desired        combination of ductility and strength is not achieved. The        maximum is defined to avoid having segregation issues which are        detrimental for the ductility. Preferably, the manganese content        is higher than or equal to 3.7%.    -   0.1%≤Si≤2.0% and 0.03%≤Al≤2.0%. According to the invention Si        and Al together play an important role: silicon delays the        precipitation of cementite upon cooling below the equilibrium        transformation temperature Ae3. Therefore, a Si addition of at        least 0.1% helps to stabilize a sufficient amount of retained        austenite. Si further provides solid solution strengthening and        retards the formation of carbides during carbon redistribution        from martensite to austenite resulting from an immediate        reheating and holding step performed after a partial martensitic        transformation. At a too high content, silicon oxides form at        the surface, which impairs the coatability of the steel.        Therefore, the Si content is less than or equal to 2.0%.

Aluminum is a very effective element for deoxidizing the steel in theliquid phase during elaboration. In addition, Al is an alpha-formerelement that increases the Ae1 and Ae3 temperatures of the steel. Thus,owing to the addition of at least 0.03% of Al, the intercritical domain(i.e. between Ae1 and Ae3) is in a temperature range favoring thepartitioning of Mn in the austenite, as described in further detailsbelow. The Al content is not higher than 2.0%, preferably not higherthan 1.2% in order to avoid the occurrence of inclusions, to avoidoxidation problems and to ensure the hardenability of the material.

The steel according to the invention must contain at least one elementchosen among niobium, titanium and vanadium, in a minimum combinedcontent of at least 0.01% Such addition will allow strengthening therecovered martensite by limiting the growth of martensitic laths throughprecipitation.

-   -   Nb≤0.080% can be added in order to refine the austenite grains        during hot-rolling and to provide precipitation strengthening.        In a preferred embodiment, the minimum amount of niobium added        is 0.010%. Above 0.080% of addition, yield strength, elongation        and hole expansion ratio are not secured at the desired level.    -   Ti≤0.080% can be added to provide precipitation strengthening.        In a preferred embodiment, the minimum amount of titanium added        is 0.010%. However, when its amount is above or equal to 0.080%,        yield strength, elongation and hole expansion ratio are not        secured at the desired level    -   V≤0.2% can be added to provide precipitation strengthening. In a        preferred embodiment, the minimum amount of vanadium added is        0.010%. However, when its amount is above or equal to 0.2%,        yield strength, elongation and hole expansion ratio are not        secured at the desired level.

The remainder of the composition of the steel is iron and impuritiesresulting from the smelting. In this respect, Ni, Cu, S, P and N atleast are considered as residual elements which are unavoidableimpurities. Therefore, their contents are less than 0.05% for Ni, 0.03%for Cu, 0.010% for S, 0.020% for P and 0.008% for N.

Some elements can optionally be added to the composition of the steelaccording to the invention:

-   -   0.1%≤Mo≤0.5%. Molybdenum increases the hardenability, stabilizes        the retained austenite thus reducing austenite decomposition        during partitioning, and reduces the central segregation which        can result from the high manganese content and which is        detrimental to the hole expansion ratio. Furthermore, Mo helps        refining the structure. Above 0.5%, the addition of Mo is costly        and ineffective in view of the properties which are sought        after.    -   0.01%≤Cr≤1% to delay the dissolution of carbides and stabilize        the retained austenite. A maximum of 1% of chromium is allowed,        above a saturation effect is noted, and adding chromium is both        useless and expensive.    -   0.0005%≤B≤0.004% in order to increase the quenchability of the        steel.

Preferably, the composition of the steel is such that the steel has acarbon equivalent Ceq lower or equal to 0.4%, the carbon equivalentbeing defined as Ceq=C %+SP/0/55+Cr %/20+Mn %/19−Al %/18+2.2*P %−3.24*B%−0.133*Mn %*Mo %.

The microstructure of the cold-rolled and heat-treated steel sheetaccording to the invention will be now described.

The cold-rolled and heat-treated steel sheet has a structure consistingof, in surface fraction:

-   -   between 10% and 30% of retained austenite, said retained        austenite being present as films having an aspect ratio of at        least 3 and as Martensite Austenite islands (so called MA        islands), less than 8% of such MA islands having a size above        0.5 μm,    -   at most 10% of fresh martensite and    -   recovered martensite containing precipitates of at least one        element chosen among niobium, titanium and vanadium.

The surface fractions and aspect ratio are determined through thefollowing method: a specimen is cut from the cold-rolled andheat-treated, polished and etched with a reagent known per se, so as toreveal the microstructure. The section is afterwards examined throughoptical or scanning electron microscope, for example with a ScanningElectron Microscope with a Field Emission Gun (“FEG-SEM”) at amagnification greater than 5000×, coupled to an Electron BackscatterDiffraction (“EBSD”) device and to a Transmission Electron Microscopy(TEM).

The determination of the surface fraction of each constituent areperformed with image analysis through a method known per se. Theretained austenite fraction is for example determined by X-raydiffraction (XRD).

The microstructure of the cold-rolled and heat-treated steel sheetincludes at least 10% of austenite which is, at room temperature,retained austenite. When present in surface fraction of at least 10%,retained austenite contributes to increasing ductility. Above 30%, therequired level of hole expansion ratio HER according to ISO 16630:2009is lower than 20%.

The retained austenite is present as films having an aspect ratio of atleast 3 and as MA (Martensite Austenite), islands less than 8% of suchMA islands having a size above 0.5 μm.

The specific minimum value of aspect ratio of the residual austenitefilms and the maximum percentage of MA islands having a size above 0.5μm have to be respected to obtain the required level of hole expansionratio HER according to ISO 16630:2009.

In a preferred embodiment, the cold-rolled and heat-treated steel sheetaccording to the invention is such that the fraction ratio between MAislands having a size above 0.5 μm and the austenite film is below 1.0or, even better, below 0.5.

In another preferred embodiment, the cold-rolled and heat-treated steelsheet according to the invention is such that less than 5% of such MAislands have a size above 0.5 μm.

In another preferred embodiment, the cold-rolled and heat-treated steelsheet according to the invention is such that the surface fraction ofaustenite films having an aspect ratio above 3 is at least 8%.

The microstructure of the cold-rolled and heat-treated steel sheetincludes at most 10% of fresh martensite. Indeed, a fraction of freshmartensite higher than 10% would lead to a hole expansion ratio HERaccording to ISO 16630:2009 lower than 20%.

In another preferred embodiment, the cold-rolled and heat-treated steelsheet according to the invention is such that the surface fraction offresh martensite is below 5%.

The microstructure of the cold-rolled and heat-treated steel sheetincludes recovered martensite containing precipitates of at least oneelement chosen among niobium, titanium and vanadium. If suchprecipitates are not present, the steel grade cannot reach the minimumvalue of tensile strength targeted by the invention.

Recovered martensite can be distinguished from fresh martensite on asection polished and etched with a reagent known per se, for exampleNital reagent, observed by Scanning Electron Microscopy (SEM) andElectron Backscatter Diffraction (EBSD).

The steel sheet according to the invention can be produced by anyappropriate manufacturing method and the man skilled in the art candefine one. It is however preferred to use the method according to theinvention comprising the following steps:

Hot rolled sheet having a thickness between, for example, 1.8 to 6 mmcan be produced by casting a steel having a composition as mentionedabove so as to obtain a slab, reheating the slab at a temperatureT_(reheat) comprised between 1150° C. and 1300° C., and hot rolling thereheated slab, the final rolling temperature being higher than Ar3, toobtain a hot rolled steel.

The final rolling temperature is preferably of at most 1000° C., inorder to avoid coarsening of the austenitic grains.

The hot-rolled steel is then cooled, at a cooling rate for examplecomprised between 1° C./s and 120° C./s, and coiled at a temperatureTcoii comprised between 20° C. and 600° C.

After the coiling, the sheet can be pickled.

The hot-rolled steel sheet is then annealed, in order to improve thecold-rollability and the toughness of the hot-rolled steel sheet, and inorder to provide a hot-rolled and annealed steel sheet which is suitablefor producing a cold-rolled and heat-treated steel sheet having highmechanical properties, in particular a high strength and a highductility.

In a preferred embodiment, the annealing performed on the hot-rolledsteel sheet is a batch annealing, performed at a temperature comprisedbetween 500° C. and 680° C., during 1000 s to 50000 s.

The hot-rolled and annealed steel sheet is then optionally pickled.

The hot-rolled and annealed steel sheet is then cold-rolled to obtain acold rolled steel sheet having a thickness that can be, for example,between 0.7 mm and 3 mm, or even better in the range of 0.8 mm to 2 mm.

The cold-rolling reduction ratio is preferably comprised between 20% and80%. Below 20%, the recrystallization during subsequent heat-treatmentis not favored, which may impair the ductility of the cold-rolled andheat-treated steel sheet. Above 80%, there is a risk of edge crackingduring cold-rolling.

The cold-rolled steel sheet is then heat treated on a continuousannealing line.

The heat treatment comprises the steps of:

-   -   reheating the cold-rolled steel sheet to a first annealing        temperature above 860° C. and maintaining the cold-rolled steel        sheet at said annealing temperature for a holding time comprised        between 30 s and 600 s, so as to obtain, upon annealing, a fully        austenitic structure,

The reheating rate to the first annealing temperature is preferablycomprised between 1° C./s and 200° C./s.

-   -   quenching the cold-rolled steel sheet at a cooling rate        comprised between 0.5° C./s and 200° C./s, to a quenching        temperature comprised between 20° C. and Ms-50° C. and        maintaining it at said quenching temperature for a holding time        comprised between 1 and 200 s,

The cooling rate is chosen so as to avoid the formation of pearlite uponcooling. For each particular composition of the steel and eachstructure, one skilled in the art knows how to determine the Ms starttransformation point of the austenite by dilatometry.

During this quenching step, the austenite partly transforms intomartensite.

If the quenching temperature is lower than 20° C., the fraction ofrecovered martensite in the final structure is too high to stabilize asufficient amount of retained austenite above 10%. Besides, if thequenching temperature is higher than Ms−50° C., the fraction ofrecovered martensite in the final structure is too low to obtain thedesired whole expansion ratio.

-   -   optionally holding the quenched sheet at the quenching        temperature for a holding time comprised between 1 s and 200 s,        preferably between 3 s and 7 s, so as to avoid the formation of        epsilon carbides in martensite, that would result in a decrease        in the elongation of the steel.    -   reheating the cold-rolled steel sheet to a second annealing        temperature comprised between TΘ and 720° C., and maintaining        the cold-rolled steel sheet at said annealing temperature for a        time comprised between 100 s and 2000 s,

During this second annealing step, the cementite gets dissolved and thecarbon and Mn diffuse from the martensite to the austenite, therebyachieving an enrichment in carbon and Mn of the austenite and recoveringthe martensite.

-   -   optionally hot-dip coating the sheet in a bath at a temperature        lower than or equal to 480° C. Any kind of coatings can be used        and in particular, zinc or zinc alloys, like zinc-nickel,        zinc-magnesium or zinc-magnesium-aluminum alloys, aluminum or        aluminum alloys, for example aluminum-silicon.    -   immediately after the second annealing step, or immediately        after the hot-dip coating step, if performed, cooling the        cold-rolled steel sheet to the room temperature, to obtain a        cold-rolled and heat treated steel sheet. The cooling rate is        preferably higher than 1° C./s, for example comprised between 2°        C./s and 20° C./s.

During this cooling step, part of the austenite may transform into freshmartensite. However, the surface fraction of the fresh martensiteremains lower than or equal to 10%, owing to the stabilization ofaustenite with carbon and manganese.

-   -   optionally, after cooling down to the room temperature, if the        hot-dip coating step has not been performed, the sheet can be        coated by electrochemical methods, for example        electro-galvanizing, or through any vacuum coating process, like        PVD or Jet Vapor Deposition. Any kind of coatings can be used        and in particular, zinc or zinc alloys, like zinc-nickel,        zinc-magnesium or zinc-magnesium-aluminum alloys. Optionally,        after coating by electro-galvanizing, the sheet may be subjected        to degassing.

EXAMPLES

Three grades, which compositions are gathered in table 1, were cast insemi-products and processed into steel sheets following the processparameters gathered in table 2, going through heating, controlled hotrolling and subsequent water cooling, achieved by quenching andself-tempering.

TABLE 1 Compositions The tested compositions are gathered in thefollowing table wherein the element contents are expressed in weightpercent: Steel C Mn Si Al Ti Nb V S P N Ae1 TΘ Ae3 A 0.146 3.86 1.480.03 — 0.059 — 0.001 0.009 0.004 645 660 780 B 0.126 5.00 0.51 1.78 —0.027 — 0.002 0.009 0.005 580 660 950 C 0.110 5.17 0.51 1.81 — — — 0.0010.017 0.005 580 660 950

Steel A and B are according to the invention whereas steel C is acomparative example.

For a given steel, one skilled in the art knows how to determine Ae1,Ae3 and TΘ temperatures through dilatometry tests and metallographyanalysis.

TABLE 2 Process parameters Steel semi-products, as cast, were reheatedat 1250° C., hot rolled and then coiled at 550° C., pickled, annealed at600° C. during 5 h, pickled and cold rolled with a 50% reduction rate.They were then processed under the following conditions: First annealingCooling Holding time at Second annealing Reheating rate Temperature Timerate Tquench Tquench Temperature Time Trial Steel (° C./s) (° C.) (s) (°C./s) (° C.) (s) (° C.) (s) 1 A 10 900 120 5 180 3 680 300 2 A 10 900120 5 180 3 680 600 3 A 10 900 120 5 210 3 680 600 4 A 10 900 120 5 30 3680 600 5 A 10 860 120 5 210 3 670 150 6 A 10 860 120 5 210 3 690 150 7A 10 820 120 5 210 3 690 150 8 B 10 950 120 5 200 3 670 600 9 B 10 950120 5 30 3 690 300 10 B 10 950 120 5 200 3 680 600 11 B 10 950 120 5 303 700 300 12 B 15 — — — — — 730 500 13 B 15 — — — — — 740 500 14 B 15 —— — — — 750 500 15 C 10 950 300 5 220 3 700 120 16 C 10 1000 120 5 220 3700 120 17 C 10 1000 120 5 220 3 680 120

The resulting samples were then analyzed and the correspondingmicrostructure elements and mechanical properties were respectivelygathered in table 3 and 4.

TABLE 3 Microstructure and precipitates The phase percentages of themicrostructures of the obtained steel sheet were determined: γ γ aspectMA FM RM RF Precipitates Trial (%) ratio (%) (%) (%) (%) in RM?  1* 14 63 2 84 0 Yes  2* 15 6 3 2 83 0 Yes  3* 14 6 3 2 84 0 Yes  4* 13 6 2 2 850 Yes  5 8 6 12 7 85 0 Yes  6 16 5 22 10 74 0 Yes  7 16 4 25 15 69 0 Yes 8* 20 11 3 1 79 0 Yes  9* 22 10 4 2 76 0 Yes  10* 20 11 3 1 79 0 Yes 11* 23 9 5 4 73 0 Yes 12 21 1.5 14 9 0 70 Yes 13 23 1.5 18 10 0 67 Yes14 23 1.5 23 14 0 63 Yes 15 20 11 2 1 79 0 No 16 20 13 2 1 79 0 No 17 1713 2 1 82 0 No *trials according to the invention. γ: stands forresidual austenite surface fraction γ aspect ratio: stands for theaspect ratio of austenite films MA: stands for MA islands surfacefraction with a size above 0.5 μm FM: stands for fresh martensitesurface fraction RM: stands for recovered martensite surface fraction RFstands for recrystallized ferrite surface fraction. Precipitates in RM:stands for presence of precipitates of Nb in recovered martensite

TABLE 4 Mechanical properties Mechanical properties of the testedsamples were determined and gathered in the following table: Trial YS(MPa) TS (MPa) UE (%) HER (%)  1* 766 1023 15.1 24.4  2* 750 1014 15.825.5  3* 722 1046 16.4 26.4  4* 774  985 15.8 35.3  5 796 1044 10.6 21   6 635 1181 14.3 17.3  7 659 1183 14.3 15.8  8* 791  924 12.3 48.2  9*760  926 14.7 46.0 10* 742  949 14.0 43.8 11* 725  967 15.3 47.4 12 8611064 18.5 16.2 13 833 1086 17.2 14.2 14 786 1110 15.4 11.6 15 559  93613.7 33   16 560  902 13.2 35   17 639  848 13.7 38   *trials accordingto the invention.

The yield strength YS, the tensile strength TS and the uniformelongation UE are measured according to ISO standard ISO 6892-1,published in October 2009. The hole expansion ratio HER is measuredaccording to ISO standard 16630:2009. Due to differences in the methodsof measure, the values of the hole expansion ratio HER according to theISO standard 16630:2009 are very different and not comparable to thevalues of the hole expansion ratio A according to the JFS T 1001 (JapanIron and Steel Federation standard).

The examples show that the steel sheets according to the invention,namely examples 1 to 4 and 8 to 11 are the only one to show all thetargeted properties thanks to their specific composition andmicrostructures.

1-17. (canceled)
 18. A cold-rolled and heat-treated steel sheet, made ofa steel having a composition comprising by weight percent: C: 0.03-0.25%Mn: 3.5-8% Si: 0.1-2.0% Al: 0.03-2.0% Ti≤0.080% Nb≤0.080% V≤0.2%V+Ti+Nb>0.01% S≤0.010% P≤0.020% N≤0.008% and optionally including atleast one of the following elements, in weight percentage: Mo: 0.1-0.5%Cr: 0.01-1% B: 0.0005-0.004% a remainder of the composition being ironand unavoidable impurities resulting from processing, the cold-rolledand heat-treated steel sheet having a microstructure consisting of, insurface fraction: between 10% and 30% of retained austenite, theretained austenite being present as austenite films having an aspectratio of at least 3 and as Martensite Austenite islands, less than 8% ofsuch Martensite Austenite islands having a size above 0.5 μm, at most10% of fresh martensite, and recovered martensite containingprecipitates of at least one element chosen from the group consistingof: niobium, titanium and vanadium.
 19. The cold-rolled and heat-treatedsteel sheet as recited in claim 18 wherein aluminium content is at most1.2%.
 20. The cold-rolled and heat-treated steel sheet as recited inclaim 18 wherein niobium content is at least 0.010%.
 21. The cold-rolledand heat-treated steel sheet as recited in claim 18 wherein carboncontent is at least 0.10%.
 22. The cold-rolled and heat-treated steelsheet as recited in claim 18 wherein a fraction ratio between theMartensite Austenite islands having the size above 0.5 μm and theaustenite films is below 1.0.
 23. The cold-rolled and heat-treated steelsheet as recited in claim 22 wherein the fraction ratio is below 0.5.24. The cold-rolled and heat-treated steel sheet as recited in claim 18wherein the surface fraction of the fresh martensite is below 5%. 25.The cold-rolled and heat-treated steel sheet as recited in claim 18wherein less than 5% of the Martensite Austenite islands have the sizeabove 0.5 μm.
 26. The cold-rolled and heat-treated steel sheet asrecited in claim 18 wherein the surface fraction of the austenite filmshaving an aspect ratio above 3 is at least 8%.
 27. The cold-rolled andheat-treated steel sheet as recited in claim 18 wherein the cold-rolledand heat-treated steel sheet is coated with Zn or a Zn alloy.
 28. Thecold-rolled and heat-treated steel sheet as recited in claim 18 whereinthe cold-rolled and heat-treated steel sheet is coated with Al or a Alalloy.
 29. The cold-rolled and heat-treated steel sheet as recited inclaim 18 wherein the cold-rolled and heat-treated steel sheet has ayield strength YS of at least 700 MPa, a tensile strength TS of at least900 MPa, a uniform elongation UE of at least 12%, and a hole expansionratio HER of at least 20%.
 30. A method for manufacturing a cold-rolledand heat-treated steel sheet, comprising the following successive steps:casting a steel so as to obtain a slab, the steel having a compositioncomprising by weight percent: C: 0.03-0.25% Mn: 3.5-8% Si: 0.1-2.0% Al:0.03-2.0% Ti≤0.080% Nb≤0.080% V≤0.2% V+Ti+Nb>0.01% S≤0.010% P≤0.020%N≤0.008% and optionally including at least one of the followingelements, in weight percentage: Mo: 0.1-0.5% Cr: 0.01-1% B:0.0005-0.004% a remainder of the composition being iron and unavoidableimpurities resulting from processing; reheating the slab at atemperature T_(reheat) between 1150° C. and 1300° C.; hot rolling thereheated slab at a temperature higher than Ar3 to obtain a hot rolledsteel sheet; coiling the hot rolled steel sheet at a coiling temperatureT_(coil) between 20° C. and 600° C.; annealing the hot rolled steelsheet, to obtain a hot-rolled and annealed steel sheet; cold rolling thehot-rolled and annealed steel sheet so as to obtain a cold rolled steelsheet; reheating the cold-rolled steel sheet to a first annealingtemperature above 860° C. and maintaining the cold-rolled steel sheet atthe first annealing temperature for a holding time between 30 s and 600s, so as to obtain, upon annealing, a fully austenitic structure;quenching the cold-rolled steel sheet at a cooling rate between 0.5°C./s and 200° C./s, to a quenching temperature between 20° C. and Ms−50°C. and maintaining the cold-rolled steel sheet at the quenchingtemperature for a holding time between 1 and 200 s; reheating thecold-rolled steel sheet to a second annealing temperature between TΘ and720° C., TΘ designating the temperature above which cementite isdissolved upon heating, and maintaining the cold-rolled steel sheet atthe second annealing temperature for a time between 100 s and 2000 s;cooling the cold-rolled steel sheet to the room temperature, to obtain acold-rolled and heat treated steel sheet.
 31. The method as recited inclaim 30 wherein the annealing of the hot-rolled steel sheet is a batchannealing, performed at a temperature 500° C. and 680° C. for 1000 s to50000 s.
 32. The method as recited in claim 30 wherein the reheatingrate of the cold rolled steel sheet to the first annealing temperatureis between 1 and 200° C./s.
 33. A method for producing a spot weldedjoint of at least two steel sheets, comprising the steps of: providingthe cold-rolled and heat-treated steel sheet manufactured according tothe method as recited in claim 30; providing a second steel sheet; spotwelding the cold-rolled and heat-treated steel sheet to the second steelsheet.
 34. The method as recited in claim 33 wherein the second steelsheet is a cold-rolled and heat-treated steel sheet manufacturedaccording to the method as recited in claim
 30. 35. A method forproducing a spot welded joint of at least two steel sheets, comprisingthe steps of: providing the cold-rolled and heat-treated steel sheet asrecited in claim 18; providing a second steel sheet; spot welding thecold-rolled and heat-treated steel sheet to the second steel sheet. 36.The method as recited in claim 35 wherein the second steel sheet is acold-rolled and heat-treated steel sheet as recited in claim 18.